Generally, there is a strict condition that a temperature distributed on the section of a duct on the entrance of an SCR (selective catalytic reduction) device located at the back of a boiler should be 20° C. or above or below to the maximum. So as to satisfy the condition, accordingly, there is a need for a structure of a duct capable of allowing the exhaust gases passing through a backpass economizer of the boiler to be mixed with each other to achieve uniform temperature distribution.
By the way, the duct structures in conventional practices have relatively small pressure losses in the mixing process of the exhaust gases having different temperatures, but they do not completely satisfy the condition that the temperature distributed on the section of the duct on the entrance of the SCR device should be 20° C. or above or below to the maximum.